Low internal doses in Minamisoma

16 August 2012

People living near to the Fukushima accident in Minamisoma have low rates of contamination and can expect correspondingly low internal radiation doses, according to the first study on internal radiation.

The coastal town of Minamisoma lies 23 kilometres to the north of the Fukushima Daiichi nuclear power plant. It suffered serious disruption in March 2011 - first from the tsunami that killed 1500 of the town's 70,000 residents, and then from the nuclear crisis that saw a significant portion of the population evacuate voluntarily. Those that stayed were advised to limit their time outdoors and remain ready for evacuation by an order that lasted six months. Part of the municipality was within the 20 kilometre evacuation zone, but this has now been relaxed to allow daylight visits.

In August 2011 around half the town's normal population was in place and researchers led by Masaharu Tsubokura of Tokyo University solicited 9498 volunteers to measure their exposure to radioactive caesium and thereby estimate their level of radiation exposure due to the substance in their bodies. Between 26 September 2011 and 31 March 2012 they were checked for the presence of caesium-134 and caesium-137 in a counter shielded from natural background radiation.

All test subjects' internal radiation doses for coming decades totalled less than 1 millisievert, apart from one dose of 1.07 millisieverts.

Caesium was only detected in 34.6% of the volunteers. In children the contamination results ranged from 2.8-57.9 becquerels per kilogram. In adults the isotopes were detected in 37.8% and found in concentrations of 2.3-196.5 becquerels per kilogram. The statistically significant difference in contamination between children and adults may have been down to factors such as greater attention to food and water intake, differences in metabolism and changes in children's outdoor activity, said the researchers.

Using conservative assumptions of acute caesium inhalation after the accident and of chronic ingestion in children, the scientists converted total caesium exposure into an estimate of committed effective dose that describes the scale internal radiation exposure over coming decades. For all the volunteers except one, these were less than 1 millisievert. The exception was one dose of 1.07 millisieverts.

The figures compare well with the typical rate of background radiation worldwide, 2.4 millisieverts per year. Currently the ambient radiation dose rate in Minamisoma town centre is around 2.9 millisieverts per year, while the government's criteria for normal habitation in an area affected by the Fukushima accident is that a person should receive no more than 20 millisieverts per year.

The scientists cautioned that it is not possible to tell if the contamination represented low ongoing exposure or a gradual reduction from previous higher levels.

In a research letter to the Journal of the American Medical Association, Tsubokura said that to his knowledge the study was the first on internal exposure to caesium from the Fukushima accident. He concluded that "no case of acute health problems has been reported so far; however, assessments of the long-term effect of radiation requires ongoing monitoring of exposure and the health conditions of the affected communities."